Abstract
Photo-assisted Ultrafast Scanning Electron Microscopy (USEM) maps the dynamics of surface photovoltages and local electric fields in semiconducting samples. Photovoltages and their gradients close to surface affect the emission yield and the detection efficiency of secondary electrons (SE), leading to photoexcited SE 2D patterns. In this work, we present a method to characterize the evolution of the patterns up to ultrafast regime. These results reveal the role of surface states in affecting the external field dynamics at picoseconds. Moreover, we show that tiny changes in surface preparation express deeply different photoexcited voltage signals. We investigate the relation between the surface chemistry of Si and photo-induced SE contrast.
Highlights
The study of dynamical Surface Photovoltages (SPV) provides information on the photophysics and charge transport in semiconductors
A multiple-pixel spatial averaging of the measured secondary electrons (SE) intensity is performed over a defined range, named hereafter area of interest (AOI)
The Ultrafast Scanning Electron Microscopy (USEM) contrast is obtained as the difference between the AOI average and the reference contrast
Summary
The study of dynamical Surface Photovoltages (SPV) provides information on the photophysics and charge transport in semiconductors. TR-SEM interpretation is based on the assumption that photoexcited spatial modulations in work function induce secondary electron (SE) contrast patterns These contrast modulations originate from the photoexcitation of local excess charge carrier distributions, which affect the configuration of surface states and the yield of SE emission. A less considered effect is that this change in charge distribution modifies the electric field outside the surface too, which in turn dynamically changes the SE collection in the SEM [1,2,4,5,6] Both mechanisms, the changes in emission yield and in collection efficiency, contribute to the 2D dynamical SE patterns. Even in the case of nominally identical samples, tiny changes in preparation and in SEM environment in dramatically different TR-SEM dynamics
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